Heat recovery method and apparatus



Dec. 3, 1968 o. A. LAAKSO HEAT RECOVERY METHOD AND APPARATUS Y 2 sheets-sheet 1 Filed June 21, 1965 MlIlj ,KAR

mvEN'roR 0L /vfe Hen/95 2l/vwo loONM i .m ATTORNEYS Dec. 3, 1968 o. A. LAAKSO HEAT RECOVERY METHOD AND APPARATUS 2 Sheets-Sheet 2 Filed June 2l, 1965 United States Patent O 3,414,038 HEAT RECOVERY METHOD AND APPARATUS Oliver Armas Laakso, Glens Falls, N.Y., assignor to Aktiebolaget Kamyr, Karlstad, Sweden Filed June 21, 1965, Ser. No. 465,635 6 Claims. (Cl. 159-17) ABSTRACT OF THE DISCLOSURE A process and apparatus are disclosed for concentrating the black liquor diluent from a cellulose digester. The diluent from the digester is flashed and the residue liquor and flash vapor are passed into indirect heat exchange in an evaporator. The released vapor from the evaporator is passed to a steaming vessel to steam wood chips fed to the digester. The partially concentrated liquor from the evaporator is passed to another flasher and the flash vapor and further concentrated liquor from the second flasher are again passed in indirect heat exchange in a second evaporator.

The present invention `relates to the digestion of cellulose. More particularly, the invention is concerned with a novel process and apparatus for black liquor extraction from the cellulose digester so as to utilize most effectively the heat in the extracted liquor.

It is conventional practice to concentrate and otherwise process the black liquor discharged from a cellulose digester in order to recover the heat therefrom and any valuable constituents which may be therein. In one such arrangement, the black liquor from the digester at elevated temperature and pressure is fed to a first flash tank where the pressure is substantially reduced Because of the pressure reduction, some of the black liquor flashes into steam and this steam is discharged from the flash tank into a steaming vessel for the purpose of preheating wood chips befone they are passed into the digester. The black liquor which remains unvaporized in the first flash tank is then fed to a second flash tank where the pressure is again dropped, usually to about atmospheric pressure. This pressure reduction causes still more of the liquor to flash into steam which is subsequently condensed and then lprocessed for turpentine recovery or the like. The liquor which remains is pumped to storage from which it may also be subsequently processed for re-use, usually after further concentration.

While the above-described system can be effectively used, it does not represent the optimum in heat recovery from the hot black liquor discharged from the digester. Additionally, while the solids concentration of the black liquor is obviously increased, it is still quite low and a higher solids concentration is desirable. Thus, for example, with black liquor of solids discharged from the digester (at 325 F.), the solids content of the black liquor discharged from the second flash tank under normal operating conditions may be only around 16.6% solids. This represents an increase of only 1.6% in the solids content of the black liquor and it is usually necessary to further concentrate the liquor if it is to be processed for chemical recovery or re-use. Additionally, the condensate obtained by condensing the flash from the second flash tank does not readily lend itself to processing for the recovery of turpentine.

The principal object of the present invention is to so modify the above described system that there is maximum use of the heat available in the black liquor discharged from the digester. A more specific object of the invention is to improve the utilization of the heat in the black liquor so that the final liquor has a higher solids content and substantially lower temperature than heretofore possible.

3,414,038 Patented Dec. 3, 1968 ice Another object is to provide certain improvements in the processing of black liquor by means of flash tanks in order that the concentrated black liquor and flash condensate discharged from the system are more readily processible for re-use or the recovery of valuable constituents therein, e.g. turpentine. n

Broadly stated, the present invention contemplates employing an evaporator after each flash tank and using the steam or vapor formed by flashing to evaporate water from the remaining liquor. More particularly, the present process includes the following steps:

(l) Passing the black liquor discharged from the cellulose digester at elevated temperature and pressure into a first flash tank operating at a lower pressure than the digester (eg. 20 to 30 p.s.i.g. `compared to 16,5 to 200 p.s.i.g. in the digester) whereby part of the liquor is flashed off as steam;

(2) Discharging the unflashed liquor from the first flash tank and feeding the same into a first evaporator operated at lower pressure than the first flash tank (eg. 15 to 20 p.s.i.g.) whereby part of the liquor in the evaporator is flashed off;

(3) Discharging from the first flash tank the steam flashed off therein `and indirectly contacting the liquor in the first evaporator with this steam whereby more of the liquor is evaporated in the first evaporator;

(4) Discharging the total amount of steam formed by flashing and evaporation in steps (2) and (3) from the first evaporator and passing the steam to a steaming vessel for steaming wood chips prior to feeding the latter to the digester;

(5) Discharging the remaining liquor from the first evaporator and passing the same into a second flash tank operated at a still lower pressure than the first evaporator whereby part of the liquor flashes off as steam in the second flash tank;

(6) Passing the unflashed liquor from the second flash tank into a second evaporator operating at a pressure lower than the pressure in the second flash tank whereby some of the liquor is flashed in the second evaporator;

(7) Indirectly contacting the liquor in the second evaporator with the steam flashed off in the second flash tank whereby a further part of the liquor is evaporated;

(8) Discharging the unevaporated liquor remaining in the second evaporator at higher solids content and lower teriperature than the black liquor fed to the first flash tank; an

(9) Condensing the vapor released in the second evaporator.

By using the flash tanks in combination with evaporators as described above, it has been found that the black liquor ultimately discharged from the system is of higher concentration, lower temperature and smaller volume than in the case where the evaporators are omitted. The discharged black liquor is consequently easier and better to handle and process. Additionally, the hnal condensate is of substantially greater volume and more preferable to handle for turpentine recovery than the corresponding condensate obtained under comparative conditions without using the evaporators as proposed.

The advantages of the present invention will be more readily apparent from the accompanying drawings wherein:

FIGURE 1 illustrates diagrammatically the system previously employed for processing digestion liquor; and

FIGURE 2 illustrates the present modification of said system.

For ease of understanding the systems of FIGURES 1 and 2 are described on the basis of a black liquor supply of 480,430 pounds per hour (at 325 F., 165-200 p.s.i.g. and 15% solids). Other quantities and temperatures recited are determined by calculation based on standard data for heats of vaporization, specific heats for black liquor at indicated temperatures and percent solids, boiling points at percent solids, etc. (e.g. Tappi, lune 1953). For convenience, specific pressures, temperatures, material quantities and other conditions are given but it will be recognized that these can be widely varied provided the present invention concepts are employed.

Referring first to FIGURE 1, it will be noted that the spent black liquor 1 from the digester is pumped to the first flash tank 2 where the pressure is abruptly reduced to 18 p.s.i.g. This results in the formation of 32,100 pounds per hour of steam (255 F.) which are fed to the steaming vessel 4 for the purpose of steaming wood chips before they are fed into the digester (not shown).

The unflashed liquor from the first flash tank 2 is then passed directly to the second flash tank 6 through conduit means 5. Second flash tank 6 is operated at a lower pressure than the first tank, e.g. atmospheric pressure p.s.i.g) as shown. This causes further flashing to occur with the formation of 15,000 pounds of steam per hour (212 F.). This steam is fed to a suitable condenser 8 where it is condensed by cooling water supplied at 10 as shown. The resulting condensate (15,000 pounds per hour) may be pumped by appropriate means 12 through a heat exchanger 14 if desired, to conventional turpentine recovery operations represented by the numeral 16.

The liquor remaining after the second flash (amounting to 433,330 pounds per hour on the basis of the present exemplication) is discharged at 16.6% solids and a temperature of 215 F. from tank 6 to the black liquor storage or other processing operations, broadly designated by the numeral 18.

Referring now to the system of the present invention, as illustrated by FIGURE 2, it is to be noted initially that the main difference over the system of FIGURE 1 is the use of first and second evaporators and 22, respectively. Each of these evaporators receives black liquor from the preceding flash tank and some of the liquor is flashed off therein because of the reduced pressures which are employed. Additionally, however, steam formed in the ash tanks is used to evaporate water from the liquor. This makes possible the increased concentration of the black liquor discharged from the system.

More specifically, as shown in FIGURE 2, spent black liquor 1 from the digester (480,430 pounds per hour at 325 F., 15% solids, 165-200 p.s.i.g.) is fed to the first flash tank 2 where the pressure is reduced to 25 p.s.i.g. The resulting flash amounts to 27,000 pounds of steam per hour and this, optionally with flash from the heat exchanger 24 of the digestion operation (3,100 pounds per hour), is fed at a temperature of 268 F. into the steam chest or heating zone of the first evaporator 20.

The liquor which remains after the first ash (453,430 pounds per hour) is discharged (at 270 F. and 15.8% solids) from the bottom of flash tank 2 and pumped by pump 41 into evaporator 20. At this point, some flashing occurs, as noted above, due to the fact that the evaporator is operated at a lower pressure (18 p.s.i.g.) than the first ash tank. Additionally, the liquor is heated by means of the steam from the flash tank 2 and, if used, the flash from the heater condensate 24. Assuming complete condensation of the flash used for heating, this results in the formation of 34,000 pounds per hour of steam (255 F., 18 p.s.i.g.) in the evaporator 20. This steam is fed to the steaming Vessel 4 to steam wood chips as aforesaid.

The liquor which remains unevaporated in the first evaporator (419,430 pounds per hour at 258 F. and 17.2% solids) is then fed into the second flash tank 6 which is operated under vacuum. This again results in flashing with the formation of 31,600 pounds per hour of steam at 173 F. This steam is fed directly to the second evaporator 22 where it is used to indirectly heat liquor discharged from flash tank 6. This liquor amounts to 387,830 pounds per hour (18.5% solids) and has a temperature of 176 F. as discharged from flash tank 6. The liquor is pumped into evaporator 22 where, because of the lower pressure in the evaporator (e.g. 20" vacuum) some flashing of the liquor occurs (4,500 pounds per hour).

An additional 32,700 pounds per hour of water is evaporated from the liquor by indirectly heating the liquor in evaporator 22 with the flash vapor from flash tank 6 and with the flash vapor from condensate from the first evaporator 20. This gives a total steam formation of 37,200 pounds per hour which is discharged from evaporator 22 and condensed in condenser 8 by means of the cooling water 10. Cooling water in the amount of 1,580 gallons per minute having an inlet temperature of F. is adequate for this purpose, the outlet temperature of the water being around F. A steam educator 25 may be utilized in association with condenser 8 as shown to ncrease the effectiveness of the condensation.

The steam used to effect the evaporation in evaporators 20 and 22 is condensed as a result of the heat exchange with the liquor in the evaporators. The condensate from evaporator 20 is pumped by pump 40 into the bottom of the second evaporator as shown by connecting line 26. With the material amounts :and other conditions used for exemplification herein, this means that 30,100 pounds per hour of condensate are fed from the first evaporator 20 to the bottom of the second evaporator 22. This condensate and the condensate resulting from the condensation of steam (31,600 pounds per hour) fed into evaporator 22 at 20 inches of vacuum as shown at 27 give a total of 61,700 pounds per hour of condensate discharged at the bottom of evaporator 22. This total condensate is pumped or otherwise fed into a condensate ash tank 28 operating at reduced pressure (e.g. 30 p.s.i.g.) with the formation of 800 pounds per hour of steam which is advantageously combined as shown by line 30 with the steam 31 from the second evaporator prior to condensing in condenser 8. The unfiashed condensate in flash tank 28, amounting to 60,900 pounds per hour at F., is then discharged at 32 and combined with the condensate 34 discharged from condenser 8. The condensate 34 amounts to 38,000 pounds per hour of condensate at 160 F. giving a total quantity of 98,900 pounds per hour when combined with the condensate 32 from tank 28. This total condensate is then fed by pump means 12, optionally but not necessarily, through heat exchange means 14, to the turpentine recovery operation 16. It is to be noted that the amount of condensate fed to the turpentine recovery using the system of FIGURE 2 is much greater than that obtained with the system of FIGURE l. In both cases, the condensate is essentially water but it includes a valuable amount of turpentine which is more readily recoverable from the condensate obtained with the system shown in FIGURE 2.

The liquor, which remains unevaporated in the second evaporator 22, is discharged from the evaporator and pumped by means 38 to the black liquor storage 18. In the example given, the liquor pumped to storage 18 amounts to 350,630 pounds per hour and has a temperature of 163 F. and a solids content of 20.2%. It will thus be noted that the solids concentration is substantially increased over that obtainable using the system of FIGURE 1 under comparative conditions (16.6% solids). The ternperature of the liquor is also markedly reduced thus showing more effective heat recovery from the black liquor starting material.

It will be appreciated that the above description of the invention, and particularly the references to specific amounts and other operating conditions such as temperatures and pressures are given only for purposes of illustration. Obviously, these conditions may be varied as desired for any particular situation, the essential features being the use of evaporators following the flash tanks and utilization of the flash from these tanks to effect evaporation from the liquor as shown.

As noted above, the use of heater condensate flash 24 with the ash from the first flash tank 2 is optional although this seems to increase the overall efliciency of the system. The heater condensate liash is obtained as follows: treating liquor (e.g. lye) is withdrawn from the heating zone of the digester, heated in a heat exchanger outside the digester and then fed back into the digester. This maintains a high heat content for the liquor. The heat exchanger is supplied with steam to heat the liquor.

' This steam condenses during the heat exchange and the resulting condensate is flashed to supply a portion of the flash vapor to the iirst evaporator optionally used herein.

Various modifications other than those described herein may be employed without deviating from the invention as defined in the following claims wherein.

I claim:

1. A process for the recovery of heat from the black liquor of a cellulose digester which comprises: (1) passing the black liquor at elevated temperature and pressure from the digester into a rst liash tank where the pressure is reduced so that part of the liquor is flashed off as steam; (2) discharging the unflashed liquor from said first liash tank into the vaporizing side of a first evaporator operated at lower pressure than said first flash tank whereby a part of said liquor is flashed off as steam; (3) passing the liquor in said first evaporator in indirect heat exchange relationship with the steam formed by flashing in said first flash tank whereby still more of said liquor is evaporated; (4) discharging the steam formed by flashing and evaporation in steps (2) and (3), respectively, from said first evaporator and utilizing said steam to steam wood chips prior to feeding the latter to said digester; (5) discharging the remaining black liquor from a first evaporator and passing the same into said second ash tank operating at a lower pressure than said first evaporator whereby part of said liquor ashes off as steam; (6) passing the liquor which remains after the flash of step (5) into the vaporizing side of a second evaporator operating at a pressure lower than said second iiash tank whereby part of the liquor is flashed olf as vapor; (7) indirectly contacting the liquor in said second evaporator with the steam flashed off in the second flash tank whereby a further part of the liquor in said second evaporator is evaporated; (8) discharging the unevaporated liquor remaining in said second evaporator at higher solids content :and lower temperature than the black liquor fed to said first flash tank; and (9) condensing the vapor released in said second evaporator.

2. The process of claim 1 wherein the condensate of step (9) is further processed for turpentine recovery.

3. The process of claim 1 wherein condensate flash 6 vapor from the digester heating system is combined with the liash vapor from the first liash tank and the -combined vapors are fed to said rst evaporator.

4. The process of claim 3 wherein essentially all of the steam fed to said first and second evaporators is condensed in said evaporators and in part subsequently ashed, the resulting ash vapor being combined with the steam formed in said second evaporator and the condensate remaining after said flash is combined with the condensate of step (9) for further processing for turpentine recovery.

5. Apparatus for processing black liquor discharged from a cellulose digester which comprises a first flash tank, means for supplying black liquor from said digester to said flash tank whereby a portion of said liquor is vaporized, means for withdrawing vapor from said flash tank and means for discharging unliashed liquor therefrom, a first evaporator receiving residue liquor and liash vapor from said first flash tankV in indirect heat exchange relationship whereby part of said liquor is evaporated and said flash vapors are condensed, a steaming vessel for preheating wood chips prior to feeding the same to said digester, means for withdrawing the released vapor from the liquor side of said evaporator and feeding same to said vessel to heat `said wood chips, means for withdrawing vunevaporated liquor from said first evaporator; a second flash tank receiving said withdrawn unevaporated liquor to flash the same; a second evaporator for evaporating, by indirect heat exchange, unflashed liquor from said `second flash tank with the flash `vapors formed in said second flash tank, means for condensing vapors evaporated in said second evaporator and means for discharging a concentrated liquor from said second evaporator.

6. Apparatus according to claim 5 including a condensate ash tank receiving condensate from said first and second evaporators.

References Cited UNITED STATES PATENTS 2,896,705 7/1959 Ramen 159-47 2,941,590 6/1960 Rosenblad 159-47 3,183,145 5/1965 Collins 162-47 3,286,763 11/1966 Jacoby 159-47 X 3,179,159 4/1965 Jafs 159-47 FOREIGN PATENTS 196,084 5/ 1965 Sweden.

NORMAN YUDKOFF, Prima/y Examiner'.

I. SOFER, Assistant Examiner. 

